1. Field of the Invention
The present invention relates to electrical wires and cables. Specifically, the present invention provides an improved electrical insulation for wires and cables and methods for producing the same.
2. Description of Related Art
One of the constant areas of concern in the production of electrical wires and cables is the need to balance high speed electrical transmission with efficient electrical insulation. One of the more effective insulative materials in this regard is insulation made from polytetrafluoroethylene (PTFE) and particularly insulation made from expanded PTFE (ePTFE), such as that disclosed in U.S. Pat. No. 3,953,566 issued Apr. 27, 1976, to Robert W. Gore. The ePTFE material has many properties which make it particular effective as electrical insulation, including a relatively low dielectric constant, chemical inertness, high strength, low electrical loss, and high use temperature (i.e. thermal stability).
Using various polymer insulation, electrical insulation is commonly produced by either extruding or wrapping an insulative material around a center conductor. Although extruding an insulation around a wire is a very rapid and effective manufacturing technique, certain extrusion processes (e.g. some RAM extrusion techniques) have a constraint that when the polymer insulation is extruded on a wire it tends to have a relatively low "pull-out" resistance, The problem of "pull out" is compounded with materials like PTFE and ePTFE due to their inherent lubricity.
As is disclosed in copending U.S. patent application Ser. No. 023,642, one promising improvement in insulation production comprises combining a PTFE material and thermoplastic expandable microspheres. Such microspheres, which are commercially available under the trademark EXPANCEL.RTM. from Nobel Industries Sweden, Sundsvall, Sweden, comprise a thermoplastic shell entrapping a volatile liquid such as isopentane. When subjected to heat or similar activation energy, the microspheres dramatically expand to many times their original size and retain such size when the activation energy is removed. By subjecting the composite of PTFE and microspheres to the required activation energy, an expanded PTFE can be produced. In addition to microsphere expansion, the PTFE can be further expanded mechanically (i.e. before, during or after microsphere expansion) to lower the density of the resulting material even further. These processes produce an electrical insulation with a very low dielectric constant and a high velocity of propagation through the conductor.
While the PTFE and expandable microsphere material can be readily co-extruded over a conductor, it has been observed that when the expandable microspheres are activated, both the inside and outside diameters of the material will increase--causing the insulation to pull away from the conductor. This condition is unacceptable and represents an extreme case of poor wire pull-out resistance.
Although a number of others have experimented with composites of polymers and microspheres to produce electrical insulation, none has suggested some ready means to produce a conductor using such insulation with strong wire pull-out resistance. Some examples of early attempts to combine polymer and microspheres for electrical insulation include: U.S. Pat. No. 4,273,806 issued Jun. 16, 1981, to Stechler (employing naturally occurring microspheres of silica and alumina in a polymer such as polyolefin or polyester); U.S. Pat. No. 5,115,103 issued May 19, 1992, to Yamanishi et al. (employing silica or polymer microspheres in an ultraviolet (u.v.) radiation curable polymer such as fluoroacrylate, silicone, or silicone acrylate); and U.S. Pat. No. 5,128,175 issued Jul. 7, 1992, to Yamanishi et al. (employing heat expandable polymer microspheres in a u.v. curable polymer such as silicone acrylate, silicone, or fluorinated acrylate). In this latter case, the expandable microspheres are expanded in flowing liquid resin, which expands the insulation in a more even manner against a conductor, but does not provide a sufficiently open-cell structure.
In Japanese Laid-Open Application JP 4,335,044, published Nov. 24, 1992, it is taught that an electrical insulation with a low dielectric constant can be produced by mixing PTFE and unexpanded thermoplastic expandable microspheres and then expanding the microspheres to create an electrical insulation around a conductor. While this material does deliver an open-cell structure with low dielectric constant, as is discussed above, co-extrusion of such material around a conductor leads to a loose attachment between the conductor and the insulation once expansion is performed in situ.
An even more serious problem in the manufacture of low dielectric cables is the preservation of low dielectric constants throughout the cable manufacturing process. This is due in large part to a typical direct correlation between dielectric constant and physical and mechanical endurance of the cable (i.e. low dielectric constant insulations are typically characterized by low resistance to mechanical force due to the high void volume and low density required to achieve low dielectric constants). As a result, if care is not exercised during the manufacture and handling of low dielectric cable, the insulative properties of the cable can be dramatically compromised before it is even placed into use.
In the past, a number of solutions have been proposed to address these concerns. For example, "kid-glove" handling techniques have been employed during the manufacture and production of electronic cable in order to reduce and regulate the forces which impact the dielectric material during cable manufacturing. Typical kid-glove techniques include: using reduced tension upon the cable components; reducing the number of rollers and similar equipment which might cause densification; and using special handling of insulation prior to wrapping of the wire or other steps which might apply load to the cable or its parts. Unfortunately nearly all such techniques tend to be costly in labor, equipment, and production throughput rates.
Another approach to limit the damage caused to low-dielectric cables during manufacture uses self-supporting covers placed over insulated conductors to bear any load encountered during subsequent processing. While some improvement may be achieved through this method, increased dielectric constant generally will still occur due to pressures placed upon the insulative material during application of the cover.
The problem of increased dielectric constant due to the cable manufacturing process itself is of particular concern in the production of multiple layered cables, such as coaxial cables. In these instances where a number of different insulative and conductive layers must be combined into a single cable, the likelihood of densification to one or more layers of insulation during this process is a distinct possibility.
Accordingly, it is a purpose of the present invention to provide an electrical cable which incorporates a low dielectric constant which is not adversely affected by the manufacturing process itself.
It is another purpose of the present invention to provide an electrical insulation of a polymer/expandable microsphere composite which snugly surrounds a conductor.
It is a further purpose of the present invention to provide an electrical insulation which can be installed quickly and easily around an electrical conductor.
It is another purpose of the present invention to provide a process for producing an insulated electrical conductor which produces a variety of high speed electrical cables, such as coaxial cables, with minimal need for special handing procedures.
It is still another purpose of the present invention to provide a process for producing an insulated electrical conductor which expands polymer electrical insulation in situ around a conductor while producing a snug fit between the conductor and the insulation.
These and other purposes of the present invention will become evident from review of the following specification.